Invalid campaign run references should not make the button clickable

[Stan]

Currently the last campaign run is saved in the user.cfg. This means that if you have a mod with another campaign, and unload it, you will get an error when clicking on continue campaign.

One solution would be to store campaign runs per mod, or to check the existence of the run before ungraying out the continue campaign button;

[wraitii]

I think I didn't do the latter because it potentially required loading a lot of stuff on the main menu, which I'd rather avoid doing.

[Stan]

Maybe we could have currentmod.currentcampaign in the config e.g. public.currentcampaign .